When the size of solid substance particles reaches the ultrafine nanometer order (called “nanoparticles” in the following), the specific surface area of the particles becomes very great, so that, even though they are solids, their interface with a gas or liquid becomes extremely large. Their surface characteristics therefore strongly affect the properties of the solid substance.
It is known that the melting point of metal nanoparticles is dramatically reduced from that in the bulk state. In comparison with conventional micrometer-order particles, therefore, metal nanoparticles offer not only fine interconnect formation capability but also other features such as low-temperature sinter capability. Owing to the low resistance and excellent weatherability of silver nanoparticles, and also their low price compared with other noble metal nanoparticles, silver nanoparticles are seen as metal nanoparticles with particular promise as the next-generation material for fine interconnects.
Known methods of manufacturing nanometer-order particles (nanoparticles) of silver are broadly divided into vapor phase methods and liquid phase methods. Vapor phase methods are ordinarily methods that conduct deposition in a gas. Patent Document 1 describes a method of vaporizing silver in a helium or other inert gas atmosphere, under a reduced pressure of around 0.5 torr. Patent Document 2 teaches a liquid phase method for obtaining a silver colloid by reducing silver ions in an aqueous phase using an amine and transferring the obtained silver deposition phase to an organic solvent phase (polymeric dispersant). Patent Document 3 describes a method in which a reducing agent (alkali metal borohydride or ammonium borohydride) is used to reduce a silver halide in a solvent in the presence of a thiol type protective agent.                Patent Document 1: JP 2001-35255A        Patent Document 2: JP 11-319538 A        Patent Document 3: JP 2003-253311A        